Technical Field
Embodiments of the invention relate generally to magnetic resonance imaging (MRI). Particular embodiments relate to cardiac MRI.
Discussion of Art
In MRI imaging, when human or other animal tissue is subjected to a uniform magnetic field, i.e., a polarizing field B0, the individual magnetic moments of particle spins in the tissue attempt to align with the polarizing field, but precess about the field in random order at their characteristic Larmor frequency. If the tissue is subjected to an RF magnetic field, i.e., excitation field B1, which defines an x-y plane and varies at a frequency near a Larmor frequency of selected particles, the net aligned moment, or “longitudinal magnetization” of those selected particles, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment. After B1 is terminated, the tipped spins “relax” back into the precession defined by B0, and, as a result, produce RF signals. Some of the relaxation RF signals vary in amplitude according to an exponential function that is characterized by a recovery time T1. T1 varies according to chemical properties of the imaged tissue. Therefore, it is possible to characterize tissue by quantifying its T1.
For cardiac applications, T1 quantification has been demonstrated as a non-invasive means for quantifying acute and chronic myocardial infarction, diffuse fibrosis, heart failure, and myocardial amyloidosis. Although T1-weighted imaging, such as myocardial-delayed enhancement (MDE), is part of routine clinical cardiac MR, an inappropriate selection of the inversion time in these methods can potentially result in poor contrast between normal and abnormal myocardium. Furthermore, quantitative analysis of T1-weighted images, such as for infarct sizing and gray zone characterization, are limited by the qualitative signal intensity variations in individual images. In comparison, T1 mapping can overcome these potential limitations and provide the underlying quantitative tissue property of the different types of normal and abnormal myocardium.
A number of approaches have recently been proposed for characterizing the physical properties of myocardium directly through T1 measurement, rather than indirectly through T1-weighted imaging. None of these approaches, however, has achieved objectively accurate results in a breath-hold acquisition. Additionally, systolic imaging has proven difficult due to challenges in providing a saturation pulse trigger delay that is short enough to permit imposing an adequate saturation pulse and recovering adequate saturation data in the short time between an R-wave trigger and the subsequent systole. With reference to EKG signal data, an R-wave is the middle and positive-going portion of a normal QRS wave within the EKG signal data; the QRS wave precedes ventricular systole.
In view of the above, it is desirable to accurately, robustly, and reproducibly quantify myocardial T1, so that images can be compared objectively between patients as well as between scanners and sites. Additionally, it is desirable to acquire a sufficient number of data points along a sufficient duration of the recovery curve to allow reliable curve fitting under any clinical imaging situation. Moreover, it is desirable to obtain adequate saturation recovery data to permit T1 mapping during cardiac systole.